Protective system for electric valve circuits



Aug. 9, 1938.

c. H. WILLIS 2,126,523

PROTECTIVE SYSTEM FOR ELECTRIC VALVE CIRCUITS Original Filed Aug. 20. 1956 Inventor:

Clodius H. Wiliis,

His Attonne g.

Patented Aug 9, 1938 Clodlns H. Willis, Princeton, N. 1., assignor to General Electric Company, a corporation of New York Application August 20, 1938, Serial No. 96,975

Renewed January 29, 1938 14 Claims. (01. 111-91) My invention relates to electric valve circuits and more particularly to control and protective systems for electric valve circuits;

While the broad aspects of my invention are generally applicable for use in connection with electric valve converting apparatus, it is particularly applicable for use in high voltage direct current power transmission systems of the type described and claimed in U. 8. Letters Patent .0 1,990,758, granted February 12, 1935 on an application of Charles W. Stone and assigned to the assignee of the present application. Briefly described, the system as disclosed in the Stone patent comprises a source of energy of constant l5 voltage alternating current which is transformed to alternating current of constant valve and then rectified by an alternating current rectifier for transmission at high voltage direct current. The constant direct current is transmitted over a transmission circuit to a receiving circuit, which includes an electric valve inverter for changing the transmitted energy to alternating current of constant'value which is transformed to alternating current 0! constant voltage for distribution purposes or for connection with another constant voltage alternating current system. The electric valve converting circuits may employ networks of the monocyclic type for transforming alternating current from constant voltage to constant current, or vice versa- These networks comprise reactances of opposite sign such as inductive reactances and capacitive reactances.

An object of my invention is to provide new and improved control and protective systems for electric valve circuits.

Another object of my invention is to provide a new and improved control system for electric valve circuits for transmitting electrical energy by means of constant current direct current.

A further object is to provide new and improved systems for protecting electric valves and the associated circuits from excessively high voltages.

In accordance with an illustrated embodiment of my invention, I provide a new control and protective system for electric valve circuits of .the type employed for transmitting electrical energy by means of high voltage direct current of constant value. In the electric systems of the type described in the above mentioned Stone patent, I connect capacitances in series relation with the constant current alternating current circuits and the associated electric valve means. These capacitances serve to assist the electric valves to regain control after irregular operation, such as loss of control, and also assist the electric valves to effect the normal predetermined current commutating function. In order to control the voltage appearing across the series capacitances, I provide a plurality of discharge paths each comprising a serially-connected resistance and inductance and each path being. connected in parallel with a dlflerent one of the capacitances.

In accordance with another illustrated embodiment of my invention, I provide an arrangement for protecting a power transmission system of the type described in the above mentioned Stone patent. A suitable switching means is arranged to short circuit the constant current direct current circuit or to short circuit the output or input circuit of an associated electric valve means in accordance with an electrical condition of a monocyclic network, such as the voltage appearing across a capacitance in the network. When the voltage of the capacitance exceeds a predetermined value, the switching means effects a short circuit of the system and the voltage is prevented from rising to excessively high values.

In accordance with another illustrated embodiment 01 my invention, I provide means for protectingelectric valve means from excessively high voltages which may exist in the system due to transient conditions. In particular, I connect in parallel with each electric valve means a voltage protective device such as a device employing .spaced electrodes, as for example arcing horns,

sphere gaps, etc. In series relation with the electric valve means to be protected, I connect an air core inductive reactance having a time constant substantially smaller than the time constant of the circuit apparatus employing iron cores. The electrodes oi. the voltage responsive devices are adjusted to eflect a short circuit of the associated electric valve means in accordance with an electrical condition of the air core reactor,

such as the voltage, appearing across the reactor, v

in this way providing an arrangement for effecting rapid and precise protection of the electric gva'lves during irregular operation.

Fora better understanding of reference may be had to the following description taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

Fig. 1 of the accompanying drawing diagrammatically illustrates the application of the various embodiments of my invention as applied to an electric valve power transmission system of the type described in the above mentioned Stone patent, and Fig. 2 showsvarlous modifications my invention,

of a simplified arrangement for controlling an electric valve circuit connected in a constant current alternating current system.

Referring now to Fig. 1 of the accompanying drawing, I have diagrammatically represented the various features of my invention as applied to an electric valve transmission system for transmitting energy between a constant voltage alternating current circuit l and a constant voltage alternating current circuit 2. For the purpose of facilitating the explanation of my invention, I have represented the electric power transmission system as being arranged to-eifect energy transfer in only one direction, that is from the alternating current circuit 1 to the alternating current circuit 2. However, it should be understood that my invention may be applied to systems in which electrical energy is transferred in either direction. A suitable means for transforming constant voltage alternating current to alternating current of constant value, such as a monocyclic network 3, is connected to the constant voltage alternating current circuit I through a suitable switching means 4. The monocyclic network 3 comprises a plurality of branches of reactances of opposite sign, such as capacitances 5 and inductances 6. Alternate junctures of reactances of opposite sign are connected to phase conductors I, 8 and 9 of the constant voltage alternating current circuit I. Each phase of a constant current alternating current circuit 10 is energized in accordance with the voltage appearing between a common juncture of reactances of opposite sign, exclusive of the junctures connected to phase conductors l, 8 and 9, and the phase conductor having a component of potential perpendicular to the vector representing the voltage impressed on the branch of the monocyclic network including the common juncture. To effect this energization of the constant current alternating current circuit I0, I employ a transformer ll having primary windings l2 and secondary windings l3 which may be connected in delta. The primary windings l2 are connected in the above described manner. This arrangement for interconnecting constant voltage alternating current circuits and constant current alternating current circuits is described and claimed in a copending application for United States Letters Patent of Burnice D. Bedford, Serial No. 97,011 filed August 20, 1936 and assigned to the assignee of the present application.

In order to convert the alternating current of constant value to direct current of constant value for electric power transmission purposes, there is provided an electric valve converting means I 4 comprising electric valves |5-20, inclusive, preferably of the type employing ionizable mediums such as gases or vapors. The electric valves l5-20, inclusive, are arranged to eifect full wave rectification of the constant current alternating current circuit.

Interposed between the constant current alternating current circuit and the electric valve converting means I4, there is arranged a plurality of.

saturable reactors 2| each connected in series relation with a different phase conductor of the constant current alternating current circuit 10 and each having a winding in series relation with a different pair of electric valves of the electric valve converting means l4. The saturable reactors 2i, whirh are provided with iron cores,

are employed to control the inductance of the circuit in series .vith the electric valves and to control the rate of change of current through the electric valves at the beginning and at the end of the conduction periods to prevent the establishment of l'igh voltage gradients within the electric valve means and thereby prevent irregular operation or failures. This arrangement for controlling electric valve means of the type employing ionizable mediums is described and claimed in a copending application for United States Letters Patent, Serial No. 97,010 of Burnice D. Bedford, filed August 20, 1936 and assigned to the assignee of the present application.

To provide a means for protecting electric valves l5-20, inclusive, from high voltages which may be established in the electric transmission system by transients or irregular operation, there is provided a plurality of voltage protective devices 22-21, inclusive, each comprising two spaced electrodes 20 and 29 and each being associated with a predetermined different one of the electric valves l5-20, inclusive. Connected in series relation with the electric valve converting means l4 and connected in a constant current direct current transmission circuit 30 energized by electric valve means i4, I provide an impedance having a substantially linear volt-ampere characteristic such as an air core inductive reactance 3|. The spacing of the electrodes 28 and 29 of voltage protective devices 22 to 21, inclusive, is adjusted so that these devices short circuit the associated electric valves when the voltage appearing across the air core inductive reactance 3| attains or exceeds a predetermined value. A suitable switch ing means 32 is employed to short circuit the output of the electric valve means i4 and hence may be employed to control the voltages existing in the system. As a further means for controlling the voltageof the electric power transmission system and for starting the system, there is provided a controllable resistance load connected across the constant current alternating current circuit l0 including resistances 33 and any suitable switching means 34 arranged to connect the resistances 33 in a Y-connection across the circuit l0. Suitable voltage protective means, such as the arcing horns 35, including spaced electrodes may be connected between the resistances 33 to limit the voltage. This arrangement for starting and con trolling an electric valve power transmission system of this type is described and claimed in a copending application for United States Letters Patent of Clodius H. Willis et al., Serial No. 70,576, filed March 24, 1936, now Patent No. 2,071,190, granted February 16, 1937, and assigned to the assignee of the present application. The constant direct current transmission circuit 30 may be grounded through suitable connections 35 if it is so desired, and the circuit 30 may be provided with suitable switching means 31 for short circuiting the circuit 30'and the circuits connected thereto. Voltage protective devices 38 are also connected across the circuit 30 to limit or control the voltage existing between the con ductors of the constant current circuit 30.

A suitable electric valve converting means 39 comprising electric valves 40-45, inclusive, preferably of the type employing ionizable mediums such as gases or vapors, is employed to invert the constant current direct current to alternating current of constant value. Each of the electric valves 40-45, inclusive, is provided with an anode 46, a cathode 41 and a control member 48. The excitation or control circuits for control members 48 01' electric valves 4045 have not been shown, but it will be well understood by thoee Skilled in the art that any'suitable control cirvery satisfactory for this purpose. An air core inductive reactance 49 is provided in series relation with the electric valve means 00 and the constant current direct current circuit 20. A plurality of voltage protective devices II, each associated with a difierent one of electric valves 40-45, are provided to control or limit the voltages appearing across these electric valves. The

protective devices 50 operate in conjunction with the air core reactance 49 to effect this control.

A constant current alternating current circuit 5i is connected to the electric valve convertingmeans 39 and supplies constant current alternating current to the m'onocyclic network 02 comprislng a plurality of branches of reactances oi opposite sign which are arranged in a manner similar to that described above in connection with the monocyclic network 3. The constant current alternating current circuit BI is connected to the monocyclic network 52 through a transformer 53 having delta-connected primary windings i4 and secondary windings It. A controllable resistance load is connected to the constant current alternating current circuit 5| ior starting the system and controlling the system voltages. This load comprises resistances 50 and 1 a suitable switching means for connecting the resistances in a Y-connection across the circuit H; The voltage across theresistances 50 is effectively controlled or limited by using-protective devices 3! including spaced electrodes.

In order to control the electric valves 40-4I,

inclusive, there is provided a plurality of capaci tances 51, 58 and 50, each 'odmected in series relation with a different phase conductor of the 1 constant current alternating current circuit 5| and in series relation with a different pair oij electric valves of the electric valve converting means 30. For example,.the capacitance I9 is connected in series relation with the electric valve 43 and is also connected in series relation with the electric valve 4,0. Capacitances 01 to 50. inclusive, serve to prevent saturation of the transformer 53 by direct current during irregular operation of electric valves 40-45, inclusive, as well as assisting in the control of the valves under normal operation. In order to control the voltage appearing across the capacitors 51-00, there is provided a suitable control means such as a plurality of shunt discharge paths each connected across a different one oi these capacitors. Each of these discharge paths may include a serially-connected resistance 00 and inductance There is also provided in series relation with the phase conductors of the constant current alternating current circuit II a plurality of saturable reactors 82 which operate in a manner similar to the saturable reactors 2| above described to control the rate of change oi current through the electric valves 40-45, inclusive. at the beginning and at the end of the conduction intervals.

In order to control the; electric power transmission system when the voltage of the system tends to exceed a predetermined range oi operating voltages, I provide any suitable means responsive to an electrical condition of the monocyclic network 52 for controlling the electric power transmission system. For example, I may mam I 0 employ any suitable voltage responsive arrangement such as a potential transformer 40 for effecting this result in accordance with the voltage 0! one oi the capacitances, such as a capacitance 04, of the monocyclic network 02. Any suitable voltage responsive device and switching means,-such as the switching device It, may be employed to eilect control of the system by short circuiting the constant current direct current circuit orby short circuiting the terminals of the electric valve converting means 30. A circuit 00 connects the potential trsnsiormer II to the device 4!. The device 00 may be adjusted so that the short circuiting oi the electric valve means 30 is effected when the voltage across the capacitance exceeds. a predetermined value. The constant potential alternating current circuit 2 may be connected to alternate iunctures oi reactances oi opposite sign of the monocyclic network 42 through any suitable switching means 01.

The operation of the electric power transmission system will be substantially the same as that described in the above mentioned Stone patant, and electrical energy will be transmitted from the alternating current circuit I to the alterhating current circuit 2. The protective system comprising the voltage protective devices 22-21, inclusive; and the cooperating air core reactance 2| will be considered first. It isto be understood that the voltage protective devices 22-2'l, inclusive, are.associated with and serve as a voltage protection means for electric valves "-20, respectively. The air core inductive reactance 2| is designed so that the time constant thereof is substantially less than'the time constant oi the other inductive apparatus in the power transmission system comprising iron cores so that the rate of rise of voltage appearing across the ter- 7 minals oi. the air core reactance 1! will be greater inclusive, and by correlating the voltage at which these devices begin to effect a-short circuit of the associated electric means to the voltage appearing across the reactance ii, I provide an arrangement whereby the electric valves are pro-- tected in a rapid and precise manner within a relatively small interval of time. The advantage portance in electric valve translating circuits where considerable iron ore inductive apparatus is employed. It, for example, abnormal operation of the electric valve converting means 04 occurs, resulting in a rapid increase in current in the constant current direct current circuit 20, the

voltage appearing. across the terminals of the air 7 core reactance 0| will'respond linearly to the rate oi change of current and will attain a predetermined value in less' time than a corresponding change in the iron core inductive apparatus, so

that there is provided an arrangement ioreiiecting protection of the electric valves within a shorter interval oi time than would bepossible by utilizing the rate of rise of voltage of iron core inductive apparatus. It is to be noted that the voltage protective devices 00 associatedyith the electric valves 40-40, inclusive, will cooperate with'the air core inductive reactance 49.to protect these electric valves in a similar manner.

, of this type of arrangement is of particular im- In systems of the type diagrammatically shown' in Fig. l where electrical energy is transmitted current circuit and wherein electric valve means are energized from a constant potential alternating current circuit through a constant current alternating current circuit, it willbe understood by those skilled in the art that as the load impedance increases, eflecting thereby a transient decrease inthe current transmitted, the system will function to tend to maintain a predetermined constant current resulting in an increase in the voltage impressed on the constant current circuit. Referring in particular to the monocyciic'network I2 associated with the electric valve converting means I! and the constant current circuit 5|, as the resistance of the load imposed on the direct current circuit 30 is increased, the monocyclic network 52 will increase the voltage impressed on the constant current circuit H. In order to afford protection and to maintain the voltage of the system within a predetermined range of operating values, the voltage responsive arrangement including the potential transformer 63 andthe switching means eflects a short circuit of the electric valve means 39 when the voltage of an element of the monocyclic network 52, such as the voltage appearing across the terminals of the capacitance 64, exceeds a predetermined value. By designing the voltage responsive switching means 6! to be responsive to a predetermined minimum value of voltage, this device will effect a short circuit of the electric valve means 13' when the voltage attains a predetermined value, thereby aiiording an effective arrangement for controlling the transmission system when the effective resistance of the constant current circuit 30 becomes exceedingly large.

The operation of the control system comprising the capacitances 51, I and it connected in series relation with the phase conductors of the constant current alternating current circuit 5i and the electric valve means 39 may be best explained by first considering the operation of the electric valve converting means 39. As will be well understood by those skilled in the art, the electric valves 40-45 will be rendered conductive by excitation circuits, not shown, to supply constant current alternating current to the circuit 5|. Each of these electric valves will be conductive for substantially electrical degrees during each cycle, but each electric valve will be conductive with any one other electric valve for only 60 electrical degrees effecting thereby the proper sequential commutation of current between the electric valves so that three phase alternating current of constant value is transmitted to the circuit Ii. The current is commutated principally from one electric valve of the group 40-, inclusive, to a predetermined other valve of the group in a predetermined order by means of the potentials of the constant current alternating current circuit 5i and hence in accordance with the electrical characteristics established by the monocyclic network 52. However, I have found that the connections of the capacitances I1, I! and 58 in series relation with the phase conductors of the constant current circuit ll assist the current commutating function of the electric valves 40-45. I have further found that the presence of the capacitances 51 to 59, inclusive, is also of material importance in connection with assisting the electric valves 40-45 to regain control during and after an abnormal or irregular operating condition, such as an arc back condition or loss-of-control condition. For example, if the electric valves 40-45 depart from the predetermined order of operation so that direct current from the direct current circuit 30 tends to flow through the constant current circuit 5| and the primary windings 54 of transformer 53, this direct current tends to saturate the core members of the transformer 53 so that the voltage of the constant current circuit Si is materially distorted which prevents this voltage from effecting the normal predetermined current commutation between the various electric valves I have found that the series connected capacitances 51 to 59 prevent this objectionable feature during abnormal operating conditions and assist the electric valve means to regain control.

Referring now to Fig. 2 of the accompanying drawing in which there is diagrammatically relation with the phase conductors of the constant current circuit 14 and serve to control the electric valves in the electric valve converting means 15 in the manner above described. In order to protect the series capacitance l6 and to control the voltage appearing across the terminals of the capacitances, I provide a plurality of discharge paths each connected across the terminals of a different one of the series capacitances. These discharge paths may comprise any suitable impedance arrangement, such as a serially-connected resistance and inductance 60 and SI. While I have chosen to represent the means for controlling the voltage across the series capacitance as comprising an impedance path, it should be understood that other suitable arrangements and devices may be employed.

The operation of the circuit arrangement dine grammatically shown in Fig. 2 may be best explained by considering the circuit when the electric valve means 14 departs from a normal predetermined plan of operation so that there is a tendency to transmit direct current through the constant current alternating current circuit 14. By virtue of the presence of the capacitance 16 in series relation with the phase conductors of the constant current alternating current circuit I4, appreciable direct current will be prevented from flowing through the primary windings 13 of transformer II and hence the voltage of the constant current circuit 14 will effect commutation of current between the electric valves of the electric valve means 15 so that normal operation may be resumed and the electric valves of this group will regain control. The parallel discharge paths comprising the serially-connected resistance 60 and inductance 6! will be effective to limit or control the voltage appearing across the terminals of the capacitances 15 by permitting a predetermined amount of the direct current to flow through the discharge paths. In this manner, the series-connected capacitances 16 are protected and at the same time the amount of direct current which flows through the primary winding 13 of transformer H may be maintained below the value which effects material distortion of the voltage of the constant current circuit "l4.

This arrangement has proved very effective in assisting the electric valves of the electric valve in 'erter to regain control after an irregular or abnormal operating condition.

While I have shown and described my invention as applied to a particular system of connections and as embodying various devices diagrammatically shown, it will be obvious to those skilled in the art that changes and modifications may be made without departing from my invention, and I, therefore, aim in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States, is:

1. In combination, a constant voltage alternating current circuit, a constant current alternating current circuit, means interposed between said circuits for transforming constant voltage alternating current to alternating current of constant value, electric valve means connected to said constant current alternating current circuit, and a capacitance connected in series relation with a phase conductor of said constant current alternating current circuit for controlling said electric valve means.

2. In combination, a constant voltage alternating current circuit, a constant current alternating current circuit, means interposed between said circuits for transforming constant voltage alternating current to alternating current of constant value, a direct current circuit for transmitting electrical energy at constant current, electric valve means interposed between said direct current circuit and said constant current alternating current circuit for transmitting energy therebetween, and means including a capacitance connected in series relation with a phase cor 1- ductor of said constant current alternating current circuit and said electric valve means for controlling said electric valve means.

3. In combination, a constant voltage alternating current circuit, a constant current alternating current circuit, means interposed between said circuits for transforming constant voltage alternating current to alternating current of constant value comprising a monocyclic network, a direct current circuit for'transmitting electrical energy at constant current, electric valve means interposed between said direct current circuit and said constant current alternating current circuit, and means including a plurality of capacitances each connected in series relation with a different one of said phase conductors of said constant current alternating current circuit and said electric valve means for preventing direct current from flowing from said direct current circuit to said alternating current circuit upon irregular operation of said electric valve means and for assisting said electric valve means to regain control.

4. In combination, a constant voltage alternating current circuit, a constant current alternating current circuit, means interposed between saidcircuits for transforming constant voltage alternating current to alternating current of constant value, a direct current circuit for transmitting electrical energy at constant current, an electric valve means interposed between said direct current circuit and said ,constant current alternating current circuit for transmitting energy therebetween, a capacitance connected in series relation with a phase conductor of said constant current alternating current circuit and said electric valve means for controlling said electric valve means, and means for controlling the voltage across said capacitance.

5. In combination, a polyphase constant voltage alternating current circuit, a polyphase constant current alternating current circuit, electric valve means interposed between said circuits for transforming constant voltage alternating current to alternating current of constant value, a direct current circuit for transmitting electrical energy at constant current, an electric valve means interposed between said direct current circuit and said constant current alternating current circuit for transmitting energy therebetween, a plurality of capacitances each connected in series relation with a different phase conductor of said constant current alternating current circuit and said electric valve means, and means for limiting the voltage appearing across said capacitances comprising a plurality of shunt discharge paths each associated with a different one of said capacitances.

6. In combination, a constant voltage alternating current circuit a constant current alternating current circuit, means for transforming constant voltage alternating current to alternating current of constant value, a direct current circuit for transmitting electrical energy at constant current, electric valve means interposed between said direct current circuit and said constant current circuit for transmitting energy therebetween, a capacitance connected in series relation with a phase conductor of said constant current alternating current circuit and said electric valve means for controlling said electric valve means, and a parallel discharge path comprising a serially-connected resistance and inductance connected across said capacitance for limiting the voltage across said capacitance.

7..In combination, an electric translating circuit, electric valve means connected in said translating circuit, an impedance having a linear volt-ampere characteristic arranged to be responsive to the current transmitted by said electric valve means, and means comprising a voltage protective device connected across saidv electric valve means for shunting said electric valve means in accordancewith the voltage across said impedance.

8. In combination, an electric translating circuit, electric valve means connected in said translating circuit, an air core inductive reactance connected in said circuit and arranged to beresponsive to the current transmitted by said electric valve means, and means comprising a voltage protective device connected across said electric valve means for shunting said electric valve means in accordance with the voltage across said air core reactance.

9. In combination, an electric translating circuit, electric valve means connected in said translating circuit, apparatus including iron core inductances connected in said translating circuit, an air core inductive reactance connected. in

-series relation with said electric valve means,

and means comprising a voltage protective device connected across said electric valve means for short circuiting said electricvalve means in accordance with the voltage appearing across said air core reactance. 1

10. In combination, an electric translating circuit comprising an electric valve means and inductive apparatus including iron cores, an air core inductive reactance connected in series relation with said electric valve means and having a time constant substantially smaller than the time constant of the apparatus including said iron cores, means for protecting said electric valve means comprising a device having spaced electrodes connected across said electric valve means and having said electrodes adjusted to short circuit said electric valve means in accordance with the voltage across said air core reactance.

11. In combination, an electric translating circuit including a constant current alternating.

current circuit, apparatus employing iron core inductive reactances, a direct current circuit for transmitting electrical energy at constant current and electric valve means interposed between said constant current alternating current circuit and said direct current circuit for transmitting energy therebetween, an air core inductive re actance connected in series relation with said electric valve means having a time constant substantially less than the time constant of said iron core inductive reactances, and protective means connected across said electric valve means comprising a plurality of spaced electrodes arranged to short circuit said electric valve means in accordance with the voltage of said air core inductive reactance.

12. In combination, a constant voltagealternating current circuit, a constant current alternating current circuit, means interposed between said circuits for transforming constant voltage alternating current to alternating current of constant value, an electric valve translating circuit connected to said constant current alternating current circuit, and means responsive to an electrical condition of said first mentioned means for short circuiting said electric valve translating circuit.

13. In combination, a constant voltage alternating current circuit, a constant current alternating current circuit, means interposed between said circuits comprising a network of the monocyclic type for transforming constant voltage alternating current to alternating current of constant value, a direct current circuit for trans mitting electrical energy at constant current, electric valve means interposed between said constant current alternating current circuit and said direct current circuit for transmitting energy therebetween, and means responsive to an electrical condition of said monocyclic network for effecting a short circuit of said direct current circuit.

14. In combination, a constant voltage alternating current circuit, a constant current alternating current circuit, means interposed between said circuits comprising a network of the monocyclic type including a plurality of branches of serially-connected inductances and capacitances for transforming constant voltage alternating current to alternating current of constant value, a constant current direct current circuit, electric valve means interposed between said constant current alternating current circuit and said direct current circuit for transmitting energy therebetween, and voltage responsive means associated with a capacitance of said monocyclic network for effecting a short circuit of said elec tric valve means when the voltage across said capacitance exceeds a predetermined value.

CLODIU S H. WILLIS.

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